After the success of Combat Evolved, a sequel was expected and highly anticipated. Bungie found inspiration in plot points and gameplay elements that had been left out of their first game, including multiplayer over the internet through Xbox Live. Time constraints forced a series of cutbacks in the size and scope of the game, including a cliffhanger ending to the game's campaign mode that left many in the studio dissatisfied. Among Halo 2's marketing efforts was an alternate reality game called "I Love Bees" that involved players solving real-world puzzles.
On release, Halo 2 was the most popular video game on Xbox Live, holding that rank until the release of Gears of War for the Xbox 360 nearly two years later. By June 20, 2006, more than 500 million games of Halo 2 had been played and more than 710 million hours have been spent playing it on Xbox Live; by May 9, 2007, this number had risen to more than five million unique players on Xbox Live As of November 2008, Halo 2 is the best-selling first-generation Xbox game with 8.46 million copies sold worldwide, with at least 6.3 million copies sold in the US alone. Critical reception of the game was generally positive, with most publications lauding the strong multiplayer component; at the same time, the campaign was the focus of criticism for its cliffhanger ending.